1. Field of the Invention
The present invention relates to a process for a chemical treatment with a phosphate, and more specifically, it relates to a process for chemical treatment by which a strong chemical film can be formed on the surface of a steel material at room temperature (or ordinary temperature).
2. Description of the Related Art
Examples of the process for a chemical treatment with a phosphate known in the art, wherein the treatment is carried out at room temperature of 40.degree. C. or less, include processes described in Japanese Unexamined Patent Publication (Kokai) Nos. 54-270478, 60-43491, 60-238486 and 63-270478.
In the process described in Japanese Unexamined Patent Publication (Kokai) No. 54-270478, the molar ratio of the phosphate ion to the metallic (zinc) ion in the treatment bath is maintained in the range of from 0.5 to 3.7, to smoothly effect a phosphate treatment at room temperature. In the process described in Japanese Unexamined Patent Publication (Kokai) No. 60-43491, the chemical treatment at room temperature becomes possible by specifying the range of the pH and the redox potential (ORP) respectively in certain ranges. In the process described in Japanese Unexamined Patent Publication (Kokai) No. 60-238486, the method of adding a nitrite ion is improved, and the nitrite ion is supplied to the treatment bath separately from a main agent to avoid the occurrence of a vigorous reaction between the nitrite ion and the main agent. In the process described in Japanese Unexamined Patent Publication (Kokai) No. 63-270478, the phosphate ion concentration (g/liter) of the phosphate chemical treatment bath composition is made lower than the active anion concentration (g/liter) to accelerate the formation of the chemical film by an immersion method at room temperature.
The phosphate chemical treatment is a process that makes the film on the surface of a metal substrate, by using the reaction between chemical agents and the metal substrate in the aqueous bath. An aqueous phosphate solution bath containing a film formable metallic ion, such as iron, manganese or zinc.
The phosphate chemical treatment process can be considered as comprising a step of etching a metal material and a step of forming a film.
The etching reaction is mainly composed of a reduction reaction of a nitrate ion or other ion as a cathode reaction, for example, EQU NO.sub.3.sup.- +2H.sup.+ +2e.fwdarw.NO.sub.2.sup.- +H.sub.2 O (endothermic reaction) (1)
and a metal dissolution reaction as an anode reaction, for example, EQU Fe.fwdarw.Fe.sup.2+ +2e (exothermic reaction) (2)
The filming formation reaction is mainly composed of a reduction reaction (as a cathode reaction) of a nitrite ion or other ion formed by the above-mentioned etching reaction, for example, EQU NO.sub.2.sup.- +2H.sup.30 +e.fwdarw.NO+H.sub.2 O (endothermic reaction) (3)
and a dehydrogenation reaction (as an anode reaction) of a phosphate ion with a metal ion, for example, EQU (Zn.sup.2+, Fe.sup.2+)+2H.sub.2 PO.sub.4.sup.- .fwdarw.(Zn, Fe).sub.3 (PO.sub.4).sub.2 +4H.sup.+ (exothermic reaction) (4)
Further, in addition to the above-mentioned reactions represented by the formulae (1) to (4), the following balance retaining reactions exist in the chemical treatment bath. EQU H.sub.3 OP.sub.4 .rarw..fwdarw.H.sub.2 PO.sub.4.sup.- +H.sup.+( 5) EQU 4OH.sup.- .fwdarw.O.sub.2 +2H.sub.2 O+4e (6) EQU NO.sub.3.sup.- +2H.sup.+ +2e .rarw..fwdarw.NO.sub.2.sup.- +H.sub.2 O (7)
In the phosphate chemical treatment process according to the present invention also, a phosphate film is basically formed on the surface of the steel according to the above-mentioned reaction.
The present inventors have investigated sludge generated in the chemical treatment bath in the phosphate chemical treatment process. In the phosphate chemical treatment process, the presence of sludge in the chemical treatment bath has been unavoidable in a room temperature treatment process, and in the high temperature heating process currently widely used in the art.
Specifically, the sludge included in the chemical treatment bath is that wherein the phosphate formed according to the above-mentioned formulae (1) to (4) does not precipitate on the surface of the steel, but forms a colloid, and further, a solid particle in the chemical treatment bath.
The sludge in the phosphate chemical treatment bath participates in the reaction represented by the formula (4) and make lower the quality of the chemical film by mixing with the film.
The formation of sludge in the phosphate chemical treatment bath means that the chemical film formable substance dissolved in the chemical treatment bath is consumed (or solidified) as sludge.
Because the sludge becomes large and grows with the lapse of time, the presence of sludge in the treatment bath, as such, serves to convert the dissolved chemical film formable ion to sludge. Specifically, the formation of sludge causes the amount of chemical film formable ion in the chemical treatment bath to be reduced and promotes the reduction in the amount of the chemical film formable ion. This causes a problem that the capability of the chemical treatment bath to form a chemical film is lowered by the reduction in the amount of the chemical film formable ion.
Further, the electro-chemical parameter controlling of the bath is hindered by the sludge existence. The formation of the sludge means that not only an originally necessary reaction system involved in the formation of the film, but also an unnecessary reaction system involved in the formation of sludge are present in the chemical treatment bath. Therefore, a state such that the sludge formation reaction is not controlled cannot be considered one in which the reaction in the chemical treatment bath is precisely controlled, and thus it cannot be considered that the film formation reaction is precisely controlled. This corresponds to that in the heat treatment process, since components of the treatment bath are always subjected to decomposition by heating to form sludge, and thus it is difficult to control the reaction in the chemical treatment bath.
Thus, in the conventional phosphate chemical treatment bath, although the necessity of a control of the amount of the sludge has been recognized, there exists no method for precisely controlling the formation of sludge.
Examples of the conventional method of controlling the amount of sludge include a method wherein the whole bath solution containing sludge withdrawn to hold in a settling tank at suitable intervals, to separate and remove the sludge, and a method wherein a liquid (or a slurry) containing sludge separated and settled at the bottom inside of the treatment bath is continually or periodically withdrawn by a pump or the like, and filtered to separate and remove the sludge. In the heating bath, however, since a large amount of sludge is formed, it is impossible to remove all of the sludge in the chemical treatment bath, so that this method is not adequate for practical use as a method of removing sludge. Further, in these methods, also in the case of a bath at room temperature, the amount of sludge cannot be sufficiently reduced.
Thus, there exists no method by which the sludge can be completely removed for a practical use.